This invention relates generally to an improved electron gun for television receiver cathode ray tubes, and is specifically addressed to an improved tetrode section, or "lower end" of such guns. This invention has applicability to guns of many types and constructions, but is believed to be most advantageously applicable to three-beam unitized electron guns for color television cathode ray tubes.
Unitized electron guns for color cathode ray tubes generate three electron beams developed by cathodic thermionic emission. The resulting beams are formed and shaped by a tandem succession of electrodes spaced along the central axis of the gun. The electrodes cause the beam to be focused on multiple phosphor groups located on the faceplate of the color cathode ray tube.
A prime objective in the design and manufacture of such electron guns is to provide small, symmetrical beam spots on the tube screen to achieve maximum picture resolution. Other desirable characteristics are an ample depth of focus and negligible tendency to arc. In addition, the aberrations that reduce definition which result from third order imagery--spherical aberration, astigmatism, and coma--should be minimal. The latter two of these aberrations are associated with positional distortion, which results when the image point, or object resolved on the screen, is off-axis, usually by reason of a physical misalignment of the gun components.
Electron guns in common use for television color cathode ray tubes consist of two discrete sections. The first is the tetrode section made up of four parts, commonly comprising (in standard terminology) the cathode, the control grid (G1), the accelerator anode (G2), and a section of the first anode (G3) of a main focus lens.
The second section of an electron gun is commonly a main focus lens, usually comprised of two or more electrodes between which are formed the electrostatic fields which serve to focus the beam and to increase the beam voltage. Each of these two sections of the electron gun, and the synergistic relationship each with the other, has been the subject of intensive study for a great many years. The bipotential and unipotential lens configurations described by Maloff and Epstein in 1938 in their text Electron Optics in Television (Mc-Graw-Hill) are still in use today. Yet advances in gun design are still being made, as shown by the extended field focus lens described and claimed in U.S. Pat. No. 3,895,253, issued to the assignee of this application.
Much attention has been addressed to the tetrode section of the gun as well as to the focus lens. The prior art shows many examples of attempts to achieve such major objectives as the developing of small, symmetrical spots to provide maximum resolution. Many attempts to improve tetrode section performance include the use of what are called "intrusion-type" electrodes; that is, one or two of the electrodes of the tetrode section have projecting from them a frusto-conical structure facing in the direction of the cathode. In Electron Optics in Television, Maloff and Epstein shown an intrusion cone-type structure on the accelerating anode (page 122). This conical structure is attached to a long cylinder for the simple purpose of diverting a beam. No prefocusing is apparently accomplished with this structure, and in general, it must be considered to be only a very early step toward the achievement of an optimized tetrode.
Other examples are found in U.S. Pat. Nos. 2,919,380; 2,484,721; 3,740,607; 3,628,077; and 3,213,311. As will become evident from the following, none of these patents teach the unique tetrode section of this invention.
An example of a tetrode section used in a unitized, in-line gun is shown by Hughes in U.S. Pat. No. 3,873,879. The unitized control grid (G1) and the screen grid (G2) consists of two closely spaced flat plates. The focus lens is the bipotential type; that is, it is a lens which presents to electrons traveling down the lens axes from the source toward the screen target, an axial potential distribution which increases monotonically from an initial low potential near the source to a final high potential. The triode section of this particular gun is characterized by having a weak prefocus; that is, a mild refraction of the beam prior to its entrance into the field of the main focus lens, and a relatively large beam half-angle, unlike the tetrode that is the subject of this disclosure.
U.S. Pat. No. 3,995,194, issued to Blacker and Schwartz and of common ownership herewith, discloses a tetrode section in an in-line gun that utilizes an extended field main focus lens. The tetrode structure is shown schematically in FIG. 1. The tetrode section is distinguished by its deliberate provision of a high penetration factor to the cathode; that is, a large measure of the field of the main focus lens is caused to penetrate the cathode G1-G2 area to affect its operation. Also, the tetrode provides little or no prefocusing. The cited patent is considered to be of interest only in that structurally it bears a superficial resemblance to the novel tetrode section described in the present disclosure.
To summarize an ideal tetrode section would provide: (1) for maximum resolution, a small, symmetrical cross-over for imaging on the screen of a color television cathode ray tube; (2) a cross-over that lies in exact coincidence with the gun axis; (3) a low G3 penetration factor to the cathode; (4) an ample depth of focus; (5) a reduced tendency to arc; (6) physical compatibility with color cathode ray tubes of the small-neck, shadow-mask type; and (7) electrical compatibility with television set circuitry. In turn, an ideal focus lens section would greatly increase beam voltage with only minimal spot size amplification, and would introduce no aberration.
Necessarily, such ideals cannot be fully attained as some of these benefits are in a measure incompatible. To cite an example: physical compatibility with a small neck requires a small gun diameter which leads to a large spherical aberration, which in turn induces a large spot size with reduced resolution. So tradeoffs and compromises must be made. Necessarily, too, the cross-over that is imaged on the screen, no matter how perfectly formed by the tetrode, can be degraded by a focus lens that introduces aberration. For example, as a class, the commonly used bipotential lens suffers from having undesirably poor spherical aberration characteristics and cannot, in a reasonably small space such as is available in a cathode ray tube neck, provide focus beam spots sufficiently small to prevent significant loss in picture resolution, prticularly at high beam current levels.